To be clear, the file system of an external hard drive won't magically change unless you reformat it. Your Seagate drive is likely NTFS-formatted all the time and an NTFS for Mac driver (e.g., Paragon) was working to allow you to write to it.
Unfortunately, once the NTFS for Mac tool stops functioning, your Seagate external hard drive will be read-only, as macOS has only read access to NTFS. This usually happens after a macOS update, which can render the NTFS for Mac driver incompatible.
The solution is simple: Switch to a compatible NTFS for Mac driver.
If you don't want your Seagate extenal hard drive to be read-only in a sudden, choose a relibale and stabe NTFS for Mac tool like iBoysoft NTFS for Mac. It's compatible with all macOS versions since macOS High Sierra and supports both Intel-based and Apple Silicon models.
Apart from allowing macOS to read and write to your NTFS-formatted drives, this tool also enables you to repair NTFS drives on Mac and format other drives to NTFS in macOS.